CVE-2024-39908 is a denial-of-service (DoS) vulnerability classified under CWE-400 (Uncontrolled Resource Consumption) affecting the Ruby REXML gem, a widely used XML toolkit in Ruby applications. Versions of REXML prior to 3.3.2 improperly handle XML inputs containing certain characters such as `<`, `0`, and `%>`, which can trigger excessive resource consumption during parsing. This flaw allows an attacker to craft malicious XML documents that, when parsed by a vulnerable REXML instance, cause the application to consume excessive CPU or memory resources, potentially leading to application slowdown or crash. The vulnerability does not affect confidentiality or integrity but impacts availability. Exploitation requires no authentication but does require that the vulnerable application parse the malicious XML, implying some level of user or system interaction. The vulnerability has a CVSS v3.1 score of 4.3 (medium severity), reflecting its limited impact scope and the requirement for user interaction. No known exploits have been reported in the wild, but the risk remains for applications exposed to untrusted XML inputs. The issue was addressed in REXML version 3.3.2, which includes patches to mitigate the resource exhaustion problem. Users unable to upgrade are advised to avoid parsing untrusted XML data to reduce risk.

For European organizations, this vulnerability primarily threatens the availability of Ruby-based applications that utilize the REXML gem for XML processing. Organizations that accept or process XML data from untrusted or external sources—such as web services, APIs, or integrations with third-party systems—are particularly vulnerable. Exploitation could lead to denial-of-service conditions, causing service outages or degraded performance, which may disrupt business operations or critical services. This is especially concerning for sectors relying on Ruby applications for customer-facing services, financial transactions, or infrastructure management. While the vulnerability does not compromise data confidentiality or integrity, the availability impact could lead to reputational damage, financial loss, and regulatory scrutiny under European data protection and operational resilience regulations. The absence of known exploits reduces immediate risk but does not eliminate the threat, especially as attackers often develop exploits following public disclosure.

The primary mitigation is to upgrade the REXML gem to version 3.3.2 or later, which contains patches addressing the uncontrolled resource consumption issue. Organizations should audit their Ruby application dependencies to identify and update vulnerable REXML versions promptly. For environments where upgrading is not immediately feasible, it is critical to implement strict input validation and sanitization to prevent parsing of untrusted or malformed XML data. Employing XML parsing libraries with built-in protections against resource exhaustion or switching to alternative XML parsers with better security profiles may also be considered. Additionally, rate limiting and resource usage monitoring on services that parse XML can help detect and mitigate potential DoS attempts. Incorporating application-layer firewalls or XML gateways that filter suspicious XML payloads can provide an additional security layer. Finally, educating developers and system administrators about the risks of parsing untrusted XML and enforcing secure coding practices will reduce exposure.